Emerging cracks forming on the ice shelf of Pine Island Glacier on Sept. 14. (European Space Agency)

An iceberg about twice the size of the District of Columbia broke off Pine Island Glacier in West Antarctica sometime between Saturday and Sunday, satellite data shows, confirming yet another in a series of increasingly frequent calving events in this rapidly warming region.

The Pine Island Glacier is one of the fastest-retreating glaciers in Antarctica, and along with the Thwaites Glacier nearby, it’s a subject of close scientific monitoring to determine whether these glaciers are in a phase of runaway melting, potentially freeing up vast inland areas of ice to flow to the sea and raising sea levels.

According to NASA, the region surrounding the Thwaites and Pine Island glaciers contains enough “highly vulnerable ice” to raise global sea levels by about four feet.

The new iceberg from Pine Island did not last long as a single chunk of ice, instead breaking off into smaller pieces that will gradually head out to sea. But this behavior is consistent with recent studies of this glacier.

The calving event resulted from two cracks that were first spotted last year using satellites.

While this calving did not give rise to a record-large iceberg, as occurred with the Larsen C ice shelf in 2016, scientists are nonetheless concerned that such events are becoming increasingly common as the glacier flows into the sea via a floating ice shelf. If the shelf destabilizes sufficiently, the glacier — like Thwaites nearby — could begin a rapid and potentially unstoppable cycle of ice loss, since the land upon which the ice rests dips downward as one heads inland.

This could allow relatively mild ocean waters to penetrate well inland, melting more ice and speeding its movement into the sea.

According to the European Space Agency (ESA), Pine Island Glacier’s ice velocity has accelerated to exceed 33 feet per day. The faster movement of ice causes the ice shelf to stretch and crack, which can cause additional ice loss. In fact, more ice appears to have broken off the ice shelf during the Sunday through Monday time frame, according to satellite imagery. Large calving events used to take place at Pine Island Glacier every four to six years, but they’re now a nearly annual occurrence.

Calving events have occurred in 1992, 1995, 2001, 2007, 2013, 2015, 2017, 2018 and now in early 2020. The 2018 iceberg was larger than this one, at about the size of Chicago.

In addition, the large cracks in the ice shelf are forming in places that scientists hadn’t seen before, such as the middle of the ice shelf.

“There have been six previous calvings from the Pine Island Glacier since 2000, and the time intervals between them have been getting smaller,” said Adrian Luckman, a geographer at Swansea University who closely follows this glacier, via email.

“These events themselves are part of the normal behavior of large glaciers with floating sections, so, whilst spectacular, this event is not significant in its own right. However, we know that, like Thwaites, the glacier has been thinning, and its shear margins have been getting weaker, all as a result of warmer ocean waters eroding the ice,” Luckman said.

As was recently confirmed at Thwaites, the Pine Island Glacier is losing mass because of a combination of factors. First, calving events, such as the one over the weekend, can suddenly move the front of the ice shelf closer to the point where the ice meets the ground below, which is a boundary known as the grounding line. In addition, the ice shelf is melting from below as relatively warm waters eat away at the underside of the ice.

Both these factors are moving the front of the ice shelf backward, toward the grounding line, which threatens to destabilize the land ice behind it.

The breaking off ice from the shelf doesn’t raise sea level because that ice was already floating — but it could lead to a quickening of ice flow into the sea, which would raise sea levels.

“The concern with Pine Island Glacier is similar to that of Thwaites,” Luckman said. “Both glaciers are sitting on bedrock way below sea level, and this deep topography extends far inland. This makes them very sensitive [to] thinning at their seaward end caused by melt from the incursion of warm ocean waters. The increased frequency of calving at Pine Island Glacier is evidence that it is responding rapidly to the warming ocean.”